This invention relates generally to pulse fusion surfacing and more particularly to an improved method and apparatus for pulse fusion surfacing. The invention also relates to a laminated knife blade produced by use of the improved pulse fusion surfacing method and apparatus.
Pulse fusion surfacing (PFS) refers to a pulsed-arc micro-welding process that uses short-duration, high-current electrical pulses to deposit an electrode material onto a metallic substrate. PFS allows a fused, metallurgically bonded coating to be applied with a sufficiently low total heat output so that the bulk substrate material remains at or near ambient temperatures. The short duration of the electrical pulse allows an extremely rapid solidification of the deposited material and results in a fine-grained, homogeneous coating that approaches an amorphous structure. The process has been used in the past to apply wear and corrosion resistant surfaces on materials used in harsh environments. Alternative coatings have been used to alter the substrate surface resistance to wear and corrosion.
Pulse fusion surfacing is a well known technique that has been used for many years and is, therefore, not described in detail. In a PFS process both the electrode and the workpiece (i.e., substrate) are conductive and form the terminal poles of a direct current power source. When a high surge of energy is applied to the electrode, a spark is generated between the electrode and the substrate. While not known for sure, it is generally assumed that a gas bubble forms about the spark discharge from the electrode and persists for a time longer than the spark itself. Metal melted due to the high temperature of the spark is then transferred from the electrode to the substrate surface via the expanding gas bubble. Alternatively, the polarities between the electrode and the substrate can be reversed so that metal can be transferred from the substrate to the electrode.
The electrode must remain moving to fracture adhesive junctions that form between the electrode and the substrate as molten metal is deposited and solidified. To prevent inadvertent adhesion, the electrode is linearly vibrated to maintain a relative motion between the two poles. Electrode vibration is created by mounting the electrode to the armature of a solenoid. Alternative methods have also been used to maintain electrode motions. For example, U.S. Pat. No. 4,405,851 to Sheldon discusses oscillating the electrode back-and-forth in a clockwise/counter-clockwise motion. In another method of maintaining relative motion between the the electrode and the substrate, the electrode is simply rotated about it axis.
Electrode vibration or oscillation, however, are not effective in generating a high quality substrate surface. Since a relatively small area of the electrode is used during the PFS process, the vibration or oscillation "flattens" or creates facets on the electrode. The electrode facets change both the physical and electrical relationship between the electrode and the substrate. For example, a fiat electrode distributes metal onto the substrate surface differently than a round electrode. In addition, the electrical characteristics (e.g., resistance) between the two poles change proportionally to the amount of electrode surface area that contacts the substrate surface. If the electrode continues to change shape over the PFS process, the characteristics of the electrode material transferred onto the substrate surface also changes.
A consistent substrate surface is critical in PFS applications where the PFS deposition layer is used to impart predetermined frictional characteristics to the substrate. For example, if the PFS process is used to increase the surface friction of the substrate, an inconsistent PFS process can actually reduce gripping efficiency. In addition, an inconsistent substrate surface is aesthetically undesirable on certain consumer products, such as cutlery. Uneven electrode wear also reduces electrode operating life, in turn, increasing PFS processing costs.
For uniform wear, the electrode can be rotated about its axis. However, constant electrode rotation makes the electrode difficult to control. For example, constant rotation causes the electrode to "run away" or pull the electrode hand held applicator away from an PFS operator. A run away condition can change the speed at which the electrode is moved over the substrate. Moving the electrode over the substrate at different speeds create an inconsistent substrate surface. If an PFS operation loses control of the electrode applicator during a "run away" condition, electrode material can be alloyed at undesirable locations on the substrate.
Spark torque is the amount of force in which the electrode strikes the substrate surface. Spark torque from electrode rotation also creates undesirable conditions on the substrate surface. For example, when the electrode is moved over the substrate surface in the same direction as the electrode rotation, minimum spark torque is created between the electrode and the substrate. However, when the electrode is moved along the substrate surface in a direction opposite from the electrode rotational direction, maximum spark torque is created between the electrode and the substrate. Since spark torque also effects the surface characteristics of the electrode deposition material, rotating the electrode during the PFS process can actually reduce substrate surface consistency.
The individual electrical discharges through the electrode to the substrate must be of short duration or a condition known as arcing occurs. Arcing is the electrical discharge of low intensity and long duration. The duration of a spark during arcing can be up to one hundred times longer than is desirable for pulse fusion surfacing purposes. A spark of this duration alters the surface characteristics of the substrate and if allowed to continue can potentially alter the structural integrity of the substrate. Spark characteristics vary depending upon the substrate material and the various PFS circuit parameters. For example, an PFS circuit that provides a pulse of sufficiently short duration for a first substrate material, may take longer to disperse onto the substrate of a less conductive substrate material. In addition, conductivity and corresponding spark characteristics change according to where the electrode is located on the substrate surface. These varying spark characteristics further reduce the surface consistency of the electrode material deposited on the substrate.
If the speed of the electrode is varied as it moves across the substrate surface, different locations on the substrate receive proportionally different amounts of electrode material. For example, if the electrode is moved slowly across the substrate, more energy and accordingly more electrode material is dispersed to the substrate surface. Alternatively, if the electrode is moved quickly over the substrate, less energy and proportionally less electrode material is transferred to the same relative area on the substrate surface.
Because of the above mentioned PFS process variations and substrate surface inconsistencies, the pulse fusion surfacing process has had limited applications. For example, known PFS processes do not produce a sufficiently consistent surface for consumer products where esthetics are material to marketability. In addition, the PFS process has had limited success in small geometry applications where finite variances in surface characteristics are less tolerable.
Accordingly, a need remains for an improved pulse fusion surfacing method and apparatus that is more widely applicable to apply a variety of surface coatings to a variety of substrates.